The present invention relates to a dual-circuit hydraulic brake system with slip control, in particular for automotive vehicles, wherein wheel brakes are connected via electromagnetically actuatable multiple-position valves to brake circuits which are hydraulically isolated from one another, wherein the brake circuits are pressurizable by a tandem master cylinder, on the one hand, and connectible to a dual-circuit pump aggregate that is adapted to be driven electromotively, on the other hand.
A hydraulic brake system comprising the preceding features is known for instance from the print "Bosch-Technische Berichte", volume 7, (1980), number 2.
The brake system described in this print comprises substantially a tandem master cylinder with a vacuum brake booster connected upstream thereof, wherein a first working chamber of the tandem master cylinder pressurizes jointly the wheel brake cylinders at the rear axle of an automotive vehicle, while the second working chamber of the tandem master cylinder serves for the pressure supply of the wheel brakes at the front axle of the automotive vehicle. Inserted into the connection between the first working chamber of the tandem master cylinder and the wheel brakes of the rear axle of the automotive vehicle is a solenoid valve controllable by slip control electronics, which valve, in the de-energized state, establishes a free passage to the first working chamber of the tandem master cylinder, while it interrupts said connection in a second position and in a third position provides for a condition in which the first working chamber of the tandem master cylinder is isolated from the wheel brakes of the rear axle of the automotive vehicle and pressure fluid is taken from the wheel brakes fed by the first working chamber.
The wheel brakes at the front axle of the automotive vehicle are supplied with pressure from the second working chamber of the tandem master cylinder, with each connection between the second working chamber of the tandem master cylinder and a wheel brake at the front axle of the automotive vehicle including an electromagnetically actuatable multiple-way valve which, in respect of its function, corresponds to the electromagnetically actuatable valve in the connection between the first working chamber of the tandem master cylinder and the wheel brakes at the rear axle of the automotive vehicle. A multiple-circuit brake system will be accomplished by a like combination, wherein the wheel brakes at the rear axle of the automotive vehicle will be controlled jointly in respect of slip, while the wheel brakes of the front axle are controllable individually.
A change of the switching position of one of the electromagnetically actuatable valves results at the same time in the start-up of an electromotively driven dual-circuit pump aggregate which during slip control in the phase of pressure build-up supplies a corresponding auxiliary pressure to the wheel brakes.
It has to be regarded as a disadvantage in the brake system described that in the phases of pressure build-up during slip control always the outlet pressure of the pump aggregate becomes effective in the wheel brake cylinders. This may have as a consequence that the braking pressure rises very quickly during the control action so that a new locked condition may occur in a correspondingly short period of time. Hence follows that relatively great pressure differences will be encountered during slip control. Besides, the outlet pressures of the dual-circuit pump aggregate will enter directly into the working chambers of the tandem master cylinder so that strong pulsation will be felt at the brake pedal during slip control what may under certain circumstances contribute to diverting the driver's attention from traffic in a braking situation which is critical anyway.
It is therefore an object of the present invention to provide a multiple-circuit hydraulic brake system with slip control comprising the features initially referred to, wherein the operation of the pump aggregate has no influence on the position of the brake pedal.